The invention relates generally to the field of photography. More specifically, the present invention relates to systems and methods for improving image quality in optical systems.
It is understood by those skilled in the art that lenses produce non-uniform illumination at a focal plane. This leads to non-uniform exposure of film, when imaging a uniformly illuminated surface. For instance, light from a uniformly lit gray wall, perpendicular to an optical axis, will pass through a lens and form an image that is brightest at the center and dims gradually as it moves radially away from the center. The intensity of light in the image will form a pattern that dims in accordance with the cos4 of the angle between the optical axis, the lens, and the point in the image plane.
In addition, other factors such as vignetting, contribute to the lens light falloff phenomena. Light falloff caused by vignetting results from mechanical interference by the lens with the path of some of the light rays as they pass through the lens and other opto-mechanical structures.
In the traditional optical photographic system, every image captured on photographic film contains some component of lens light falloff. The lens of an optical printer also introduces lens light falloff during the printing process. However, in the case of negative photographic film, this falloff occurs to the negative of the original scene, and thus has the effect of counteracting the lens light falloff in the negative and providing a means of partial falloff compensation. In the case of reversal films, the lens of the optical printer creates more lens light falloff, thus exacerbating the effect.
In general, the level of falloff compensation counteracted by the optical printer for negative film images is less than the amount of falloff caused by the optics of a capture device. For this reason, there is a need to compensate for residual falloff induced by a capture system on a captured scene.
In U.S. patent application Ser. No. 09/293,197 filed Apr. 16, 1999, which is entitled xe2x80x9cA Method for Compensating Digital Images for Light Falloff and an Apparatus Thereforxe2x80x9d, invented by the present inventors as the present application and assigned to the present assignee, the teachings of which are incorporated herein by reference, describe a method of correcting for lens light fall of in a digital photographic copier. In the case of a digital image, manipulation ability is enhanced by the fact that the images reside in digital form in a computer so that digital manipulation is readily available on a pixel by pixel basis.
However, in the case of an image created by a traditional film based photographic system, the approach involving digital manipulation of every picture element is not always practical or feasible. Consequently, a need exists to compensate for lens falloff in a captured scene where the image resides on traditional photographic film materials.
In another aspect of photography and illumination of scenes captured thereby, when a flash is used in order to provide more illumination on a scene than is available from ambient lighting, an effect similar to lens light falloff is often observed in the captured image. This is due to the fact that most electronic flash units do not provide uniform illumination over the image field, and in particular where the lens angle of view is at or greater than the flash units designed angle of coverage. The effect is to reduce the level of illumination near the edges of the scene, in a similar fashion to lens light falloff. Consequently, there exists a need to create a flash light falloff compensation for those scenes where flash falloff degrades image quality.
In U.S. Pat. Nos. 5,926,258; 5,155,524; 5,467,165; and 5,638,153 all assigned commonly to AGFA-Gavaert, a photographic copier with a masking unit positioned in the optical path is described. This masking unit is described in terms of an array of LCD elements, each of which may assume different shades of gray. In the aforementioned U.S. patents, the unsharp mask gray values are derived from the density values of the image and superimposed onto the masking unit for the purpose of compressing the dynamic range of the image such that the optical printer can adapt the contrast range of the captured image to the contrast range available with the printed medium and therefore produce an image that preserves as much detail from the original scene as is practicable. The process employed in these patents is to mask the image in strict accordance with the illumination values found in the digital image data. This is referred to generally as a contrast mask and is not designed to be effective, nor is it effective, for compensating for lens light falloff or flash light falloff.
Consequently, a need remains in the art for a system or method for compensating for lens light falloff and flash light falloff in an optical photographic copying process.
The need in the art is addressed by the apparatus and methods of the present invention. The illustrative embodiments include an apparatus for compensating for light falloff present in a photographic master frame at the time the master frame is printed in an optical photographic copier. The apparatus includes a masking unit that has a plurality pixel elements, each controllable in light transmittance. The controller is operable to adjust the transmittance of the plurality of pixel elements in accordance with a light falloff function. In one embodiment, a control input for specifying one of a plurality of light falloff compensation functions defining a light falloff correction parameter for the master frame is provided. The controller then operates in accordance with the light falloff function specified by the control input.
The illustrative embodiments also include a method of compensating for light falloff present in a photographic master frame at the time the master frame is printed to a copy in an optical photographic copier that has a masking unit having a plurality of individually controllable pixel elements, a controller, and a control input. This is accomplished by receiving an input parameter by the control input, calculating a light falloff correction parameter by the controller based on said input parameter, generating an individual compensation value for at least one controllable pixel elements based on said correction parameter, and superimposing the compensation value onto the master unit so that the light falloff in the photographic copy is reduced.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.